1. Field of the Invention
The present invention relates to an LED driving device, and more particularly, to an LED driving device employing a boost type direct current (DC)-DC converter, capable of protecting an LED and a driving circuit from an overvoltage loaded on the LED and controlling a duty of an output signal of a current-mode pulse with modulation (PWM) driving IC employed by the boost type DC-DC converter to be perfect 0%.
2. Description of the Related Art
In general, since a cold cathode fluorescent lamp (CCFL) used for a light source of a backlight of a liquid crystal display (LCD) uses mercury gas, environmental pollution may be caused, response speed is low, color reproduction characteristics is low, and it is difficult to reduce weight, thickness, and size of an LCD panel.
On the other hand, light emitting diodes (LEDs) are environmental, have a high response speed of several nano seconds, are effective on a video signal stream, can be impulsively driven, have color reproduction characteristics of 100%, whose brightness and color temperature can be freely changed by controlling the amount of light of red, green, and blue LEDs, and have advantages on the reducing weight, thickness, and size of an LCD panel. Therefore, currently, LEDs are positively employed for a light source of a backlight of an LCD panel.
When a plurality of LEDs is connected in series to be used in an LCD backlight employing LEDs, a driving circuit capable of supplying a static current to the LEDs and a dimming circuit for controlling brightness of LEDs to freely control the brightness and color temperature or compensate temperature. FIG. 1 illustrates a conventional LED driving circuit.
FIG. 1 is a circuit diagram illustrating a conventional LED driving circuit 10 employing a general boost type DC-DC converter. Referring to FIG. 1, in the LED driving circuit 10 employing the DC-DC converter, an inductor L and a diode D are connected to a + terminal of a DC power Vin in series and a capacitor C and an LED array 11 are mutually connected in parallel between the diode D and a − terminal of the DC power Vin. A switch 12 and a voltage detection resistor Rs are connected in series between the inductor L, a connection node of the diode D, and the − terminal of the DC power Vin. A voltage value detected by the voltage detection resistor Rs is inputted to a PWM driving portion 13. The PWM driving portion 13 controls a duty ratio of turning on-off of the switch 12, depending on the detected voltage value. A MOSFET may be used for the switch 12 by controlling a gate voltage of the MOSFET, as shown in FIG. 1.
When the switch 12 is turned on, a voltage applied to the DC power Vin flows into the inductor L and the switch 12 and energy is accumulated in the inductor L. When the switch 12 is turned off, a total amount of the DC power Vin and the energy accumulated in the inductor L passes the diode D and is applied to the LED array 11. In this case, a voltage applied to the LED array 11 is smoothed by a smoothing capacitor C and a value of the voltage is not less than an input voltage Vin.
In the conventional LED driving circuit 10 employing the boost type DC-DC converter, brightness of the LED array 11 is controlled by controlling the voltage value detected by the voltage detection resistor Rs by controlling a resistor value of the voltage detection resistor Rs. Though the brightness can be controlled by using a variable resistor for a voltage detection resistor Rs, since a wattage resistor has to be used as the voltage detection resistor Rs due to a high Led resistance, variableness is difficult. Also, when a plurality of LEDs emitting light of various colors is used, a different driving circuit for each color of LEDs is used. Since a variation occurs in a value of the voltage detection resistor Rs for each driving circuit, controlling brightness, color coordinates, and uniformity is difficult.
Also, in the LED driving circuit 10 employing the boost type DC-DC converter, when a load, namely, the LED array 11 is open or a voltage of the inductor L instantly increases more than the load, an overvoltage may be applied to the load and the LED driving circuit 10 employing the boost type DC-DC converter and the LED array 11 may be damaged. Accordingly, an overvoltage protection circuit is essentially in the LED driving circuit 10 employing the boost type DC-DC converter.
Also, a static current type pulse current is used for a driving power supplied to from the LED driving circuit 10 employing the boost type DC-DC converter to the LED array 11, and a PWM control method of controlling a duty of the pulse current is applied to control the brightness and color of the LED array 11. Namely, the PWM driving portion 13 is used for control an on-off duty of the switch 12 via PWM control. Generally, a current-mode PWM driving IC is used as the PWM driving portion 13. However, in the LED driving circuit 10 employing the boost type DC-DC converter, since a duty of an output signal does not perfectly become 0% due to characteristics of the current-mode PWM driving IC, LEDs are slightly lighted when to turn off LEDs.